1. Field of the Invention
The present invention relates to a molten metal pouring pipe for a pressure-casting machine.
2. The Prior Art
A pressure-casting machine is known as an apparatus for casting molten metal.
A conventional pressure-casting machine 35 is described below with reference to FIG. 7.
FIG. 7 is a schematic vertical sectional view illustrating a conventional pressure-casting machine 35. As shown in FIG. 7, the conventional pressure-casting machine 35 comprises:
a mold 36 having an opening 36b in a bottom wall 36a thereof;
a closed vessel 37, arranged below said opening 36b of said mold 36, having a lid 37a and a compressed gas supply port 37b;
a ladle 38 arranged in said closed vessel 37;
a molten metal pouring pipe A made of a refractory, having a sufficient length such that an upper end thereof is attached substantially vertically from below to said opening 36b in said bottom wall 36a of said mold 36, and a lower end thereof runs through said lid 37a of said closed vessel 37 and is immersed into molten metal received in said ladle 38 arranged in said closed vessel 37; and
a compressed gas supply means (not shown) for supplying a compressed inert gas into said closed vessel 37 through said compressed gas supply port 37b of said closed vessel 37, so as to pour molten metal received in said ladle 37 into said mold 36 through said molten metal pouring pipe A.
According to the above-mentioned conventional pressure-casting machine 35, unlike the case of pouring molten metal into the mold from above, splash of molten metal never adheres onto an inner surface of the mold 36, and as a result, it is possible to manufacture a metal cast strand having a smooth surface. Since it is also possible to pour molten metal received in the ladle 38 into the mold 36 through the molten metal pouring pipe A without causing molten metal to be in contact with the open air, the oxidation of molten metal can substantially perfectly prevented, thus permitting the manufacture of a metal cast strand the chemical composition of which is very strictly controlled.
In the above-mentioned pressure-casting machine 35, there is a demand for a scaling up of the mold 36 and the ladle 38 with a view to improving the manufacturing efficiency, and along with this, it is inevitable to use a large-sized molten metal pouring pipe having a length of at least 4 m and an outside diameter of at least 350 mm.
A molten metal pouring pipe is used under very severe conditions. More specifically, not only the molten metal pouring pipe comes into contact with a high-temperature molten metal, but also molten metal having a very high pressure passes through a bore of the molten metal pouring pipe. The molten metal pouring pipe is therefore made of a refractory excellent in spalling resistance and having a high strength, for example, a refractory comprising aluminum oxide (Al.sub.2 O.sub.3), carbon (C) and/or silicon oxide (SiO.sub.2).
However because an article made of a refractory is limited in size, it is difficult to integrally form the above-mentioned large-sized molten metal pouring pipe with a refractory.
Under such circumstances, a large-sized molten metal pouring pipe for solving the above-mentioned problems is known (hereinafter referred to as the "prior art"). The molten metal pouring pipe B of the prior art for a pressure-casting machine is described below with reference to FIGS. 1 and 2.
FIG. 1 is a schematic front view illustrating a molten metal pouring pipe B of the prior art for a pressure-casting machine, and FIG. 2 is a schematic partial vertical sectional view illustrating the molten metal pouring pipe B of the prior art shown in FIG. 1.
As shown in FIGS. 1 and 2, the molten metal pouring pipe B of the prior art for the pressure-casting machine comprises:
two pipe sections 1 and 6 made of a refractory, connected to each other in series and in a liquid-tight manner by means of a threaded joint b, the threaded joint b comprising (i) a male screw 5 formed on a cylindrical outer surface of a lower end portion 3 of an upper pipe section 1 out of the two pipe sections 1 and 6, and (ii) a female screw 10, with which the male screw 5 of the upper pipe section 1 is to engage, formed on a cylindrical inner surface of an upper end portion 7 of a lower pipe section 6 out of the two pipe sections 1 and 6.
The upper pipe section 1 and the lower pipe section 6 are connected to each other in series and in a liquid-tight manner by causing the male screw 5 of the lower end portion 3 of the upper pipe section 1 to engage with the female screw 10 of the upper end portion 7 of the lower pipe section 6, whereby a bore 4 of the upper pipe section 1 and a bore 9 of the lower pipe section 6 communicate with each other. When causing the male screw 5 of the lower end portion 3 of the upper pipe section 1 to engage with the female screw 10 of the upper end portion 7 of the lower pipe section 6, it is possible to further improve liquid tightness at the junction between the upper pipe section 1 and the lower pipe section 6, i.e., at the threaded joint b, by applying a refractory mortar on the surfaces of the male screw 5 and the female screw 10.
According to the above-mentioned prior art, the large-sized molten metal pouring pipe B having a total length of at least 4 m and an outside diameter of at least 350 mm can be provided by connecting the two pipe sections 1 and 6 made of a refractory.
However, the molten metal pouring pipe B of the prior art has the following problems: When applying the molten metal pouring pipe B of the prior art to the pressure-casting machine to cast molten metal, the molten metal pouring pipe B is subjected to a considerable stress by molten metal passing therethrough, and particularly, stress is concentrated on a portion 3a near the starting point of the male screw 5 of the lower end portion 3 of the upper pipe section 1, and on a portion 7a near the starting point of the female screw 10 of the upper end portion 7 of the lower pipe section 6. Such concentration of stress may cause cracks in the above-mentioned portions 3a and 7a to cause the leakage of molten metal, and furthermore, the molten metal pouring pipe B may be broken at the junction between the upper pipe section 1 and the lower pipe section 6, i.e., at the threaded joint b, thus resulting in a stoppage of the casting operation.
Under such circumstances, there is a strong demand for the development of a molten metal pouring pipe for a pressure-casting machine, which increases the strength at the junction between the upper pipe section and the lower pipe section, thereby making it possible to continue a stable casting operation for a long period of time, but such a molten metal pouring pipe has not as yet been proposed.